For many years roof bolts have been used for supporting rock strata above the roofs in underground mines. Roof bolts are typically formed of elongated rods of steel anchored in boreholes by (1) various types of mechanical shells or (2) resin which extends along all or part of the length of the bolt. More recently, bolts which are both mechanically anchored in the borehole and reinforced by resin have proved advantageous by preventing loss of tension (e.g., bleed-off) in the bolt over time which had previously been caused by deterioration of surrounding strata, creep, improper hole size or bolt damage during installation.
Within the past 10 years, significant advances in bolt technology have contributed to faster installation speeds, greater anchoring effectiveness, and more favorable economics in the use of roof bolts. One example is a roof bolt such as the one known as the INSTAL bolt made by Jennmar Corporation which operates to mix a two-part resin and then tension the bolt by continuous rotation in a single direction immediately after the bolt is inserted into the borehole. See, for example, U.S. Pat. Nos. 4,413,930 and 4,419,805. Another advance was the use of an effective compression ring for compacting resin into a solid, void-free column which surrounds and extends below an expansion shell which provides better anchorage with less resin. See U.S. Pat. No. 4,865,489.
Another advance was a technique for reducing internal friction between various component parts of a roof bolt, for example, a low friction washer formed of plastic or lubricated components, located between the bearing plate, which is adjacent to the mine roof, and the hardened washer which is positioned adjacent to the forged head on a roof bolt.
These latter devices are particularly effective in reducing internal friction and frictional torque loss during bolt installation since the interface between the bearing plate and hardened washer is believed to be the greatest single source of internal friction in a roof bolt. Other developments include adjusting angles and the surface areas between the camming plug portion of an expansion shell and the adjacent expansion fingers and to provide low friction materials between those adjacent surfaces.
Such efforts to reduce internal friction are known to increase the tension/torque ratio from about 50:1 in a bolt where the hardened steel washer engages the bearing plate, to as high as 120:1. While the installed load of a roof bolt is generally not recommended to exceed 70 percent of the yield strength of the bolt material, higher tension/torque ratios can be obtained by upgrading bolt specifications. Although the reduction of internal friction in various components of a roof bolt was generally considered advantageous, the effect of increasing the installed load and consequentially the tension/torque ratio for various types of roof strata was not understood.